With the tube amps I’ve owned that can switch between the two modes of operation I always ended up preferring Ultralinear mode,mirroring your findings of better imaging definition,slightly forward staging & more "meat"on the bones in the bass & upper bass...I do however believe this is speaker dependent..All of the speakers I had at the time were already naturally warm sounding(Harbeth M30.1,Fritz Carbon 7SEMk.II & Sonus Faber Lumina III),negating the benifits Triode mode provides...With a speaker that has some tendency towards a more lit up-forward tone(Triangle-Kef-Klipsch or anything with metal drivers) I think Triode mode might be the way to go...
I hate to say it, but now I think maybe I like my amp in ultralinear mode versus triode
It's a Cary V-12; it features a dozen EL34s and each pair has a switch in between them that configures that pair to either triode or ultralinear. In full triode Cary listed in the specs that it makes 50 wpc and in full ultralinear 100 wpc. For most of the twenty three years that I have owned this amp I have always felt that I preferred triode except for the occasions that I wanted to full out blast (it has literally been many years since I've felt the need to full out blast).
However, today I experimented with a couple of things in my system, and after listening to the same "Jazz Essentials" (compilation) red book CD a couple of times all the way through, the next thing I experimented with was switching to full ultralinear.
Maybe there was more "PRaT"? (Which is a term I am still not sure that I completely grasp.) Maybe . . . but what I do feel I noted for sure was that the imaging (particularly the imaging in the center) had more weight (meatier?) and was presented more forward, which I actually like.
I put a few more hours in (one more time with Jazz Essentials, Holly Cole/It Happened One Night, Dave's True Story/Sex Without Bodies, selected tracks from Rebecca Pigeon/The Raven and Once Blue/self titled and Norah Jones/Feels Like Home) after switching to ultralinear. (No booze during this session, just coffee.) The jury is still out on this, but I do have some CDs in mind that I want to listen to over the next few days as I continue to evaluate.